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Building Sustainably » Treatments and Finishes » Exterior Finishes
Decorative Coatings
Decorative coatings for exterior timber are of three types: paints, varnishes and exterior wood stains. Preservative treatments protect timber against decay and insect attack but do not themselves provide a decorative surface.
Modern Coatings
Modern coatings for timber and timber components have shown dramatic developments in recent years and some joinery manufacturers using factory-applied coatings now offer ten-year maintenance-free (except for cleaning) guarantees for their products.
Exterior wood stains
Exterior wood stains offer the best options for decorative and protective treatment for exterior timber. They are all translucent, although with some of the darker colours, the grain pattern of the wood can only be seen with difficulty. They have two major advantages for use with timber:
- they allow the timber to 'breathe' ie water vapour can pass in and out of the timber, but they offer greater resistance to the passage of liquid water
- their maintenance schedule usually consists of a wash-down to remove dirt and loose particles before recoating
They are available in a wide range of types and colours. Although many products contain preservative chemicals, these are to prevent mould growth on the finish itself. They do not provide preservative protection for non-durable timbers which should be treated with preservatives if required.
Paint
Paint includes conventional gloss paints and also newer opaque products which possess some moisture permeability and which have more in common with opaque wood stains. Surface preparation before painting is critical and maintenance may require the removal of old, broken down paint before recoating.
Varnishes
Varnishes are essentially paints without pigment. Like paints, they form a film on the surface of the timber. Although they can provide an attractive appearance, varnishes require high levels of preparation and maintenance and are not generally recommended for exterior woodwork.
